The guidelines for foundries designing rigging for sand castings have been available for quite some time — and are well accepted. These rules can be adapted and used successfully to rig castings made by investment casting, too. When combined with casting simulation software, effectively rigging investment castings to produce sound parts is a fast and repeatable process. Here’s how:
The design process. The general design process consists of these four steps:
1. Simulation of the ‘naked’ casting.
2. Gate sizing and feeding design.
3. Rigging geometry creation.
4. Verification via CFD/solidification simulation.
The first step in the rigging process is to run a simulation of the part ‘naked’; i.e., without any rigging system. Simulation results show the effects of the part geometry on the overall solidification. In this simulation, filling analysis typically is not done, which provides extremely rapid results and can point out preferred gate locations that will promote directional solidification.
All that is required for the initial simulation is a casting model, normally provided by the customer in STL file format, plus the basic process details, such as casting alloy, shell material, pouring temperature, and shell pre-heat temperature.
Gate and feeder-bar design. The data from the un-rigged simulation can be used to design the rigging components. Typically, the gates are designed first, followed by the feeder bar. The software uses the progression of solidification, along with a pattern recognition algorithm, to determine the separate feeding paths on the casting. The software can find the last points to freeze on each feeding path, which are the preferred gate contact points.
Gate and feeder-bar sizes for each feeding zone are calculated using variations on the well-known “Modulus technique.” While the Modulus is a geometric calculation (Volume/Surface Area), solidification time information from the simulation is converted into a “Thermal Modulus.” This takes into account not only casting alloy and shell material, but also the solidification dynamics of the specific situation, including use of insulating materials such as Kaowool or Fiberfrax wrapping.
These are guidelines for gate and feeder bar sizing
• From the Riser Design Wizard, calculate the maximum modulus of the feeding zone.
• The 2D modulus of the casting end of the gate will be equal to the maximum modulus.
• The 2D modulus of the feeder-bar end of the gate will be 1.2-times the maximum modulus.
• The 2D modulus of the feeder bar also will be 1.2-times the maximum modulus.
• For the square cross-section, the modulus is the edge length/4.
Once we know the maximum modulus in the casting or the feeding zone, we can calculate the appropriate size for a tapered gate, as well as feeder-bar dimensions that will adequately feed that part of the casting. This is done in the Riser Design Wizard, which was designed originally to calculate cylindrical risers for the sand casting process. However, it provides good information for investment castings, too.
An example of the wizard screen shows the modulus calculations used to size both the tapered gate and the feeder bar.
Modeling the rigging system. Gate and feeder-bar calculations normally take only a few minutes to perform. Rigging components can be created in CAD or in the simulation software itself. Items that will be used for more than one casting, such as a standard size of pouring cup, can be created in a component format, and re-used as needed, thereby saving considerable time in the model creation phase.
If a library of gating components is developed and used the entire rigging design process, from loading the unrigged model to having a fully rigged geometry ready for verification simulation, can be as short as 30 minutes or so.
While investment casting can be more time-intensive than many sand casting sequences, casting simulation software can help those foundries produce high-value components quickly and repeatably.
David Schmidt is the vice president of Finite Solutions Inc. Contact him at [email protected]